1. Field of the Invention
The present invention is directed to a double surface coil for a nuclear magnetic resonance apparatus of the type having two individual coils each consisting of two sub-coils connected in parallel.
2. Description of the Prior Art
The use of surface coils is useful in a nuclear magnetic resonance apparatus for generating a tomogram (magnetic resonance imaging, or MRI) as well as for conducting spectroscopic examinations. Double surface coils of the type having two individual coils, each consisting of two sub-coils, are known in the art and are particularly suitable for obtaining a mammogram.
The use of such surface coils is particularly advantageous if certain parts of the examination subject are to be interpreted with an improved signal-to-noise ratio. Transmission is usually undertaken with a whole-body antenna, and reception is undertaken with the surface coil. When transmitting using the whole-body antenna, the surface coil must be decoupled from the system so that it does not result in topical elevations of the field in the resonant condition, and thus to a local heating of the examination subject. Given surface coils which are composed of a plurality of individual coils, any individual coil which is not active during the reception mode must be decoupled during the reception mode so that the signal-to-noise ratio is not degraded. A surface coil is disclosed in European Application 0 280 908 wherein switches are provided for this purpose, the switches being in the form, for example, of PIN diode switch which disconnect the sub-coils which are not required for a particular use.
Decoupling of a whole-body antenna using a quarter-wave cable is disclosed in European Application 0 262 495. The quarter-wave cable is connected to the surface coil. The end of the quarter-wave cable is shorted when emitting high-frequency pulses via the whole-body antenna, and is opened when receiving nuclear magnetic resonance signals with the surface coil. The whole-body antenna is thus decoupled while the surface coil is receiving.